Prior art jars are disclosed by U.S. Pat. Nos. 2,989,132 and 4,261,427. The jars disclosed in my prior U.S. patents permit an upward jarring, snap action blow to be administered to the lodged tools in the drilled bore of a well by stretching the drill string, the jar thereafter releasing and permitting the recoiling drill string to impose an upward jarring blow to the tools through the jar. While my prior jars fulfill this requirement well, the time at which the upward jarring blow is administered to the lodged tools is affected by not only environmental factors but also the stress in the drill string imposed by the drill rig operator. The environmental factors include the viscosity of the oil used in the jar, the temperature of that oil and the state of wear of the jar. The jar of the present invention includes design features which make it essentially immune to environmental conditions such as the temperature of the jar in the well.